In my own point of view, gambling is not a sin if you can control yourself when you play and if you are not an addict in gambling because when you only play, you only play on it not making it as a hobby that will come up being addicted on it. That's why it is better to limit yourself when you are playing.
And if you can't control yourself and you gamble that's a sin? I don't think sin starts there, it is starting when you are not comfortable anymore with your financial holdings. And what you do is you are gambling just to seek your satisfaction and have some more money through it.